1. Field of the Invention
This invention relates to electronic components such as transformers, choke coils etc. in which the coil element formed by winding the coil around a toroidal core is entirely encased in a resin case.
2. Prior Art
FIG. 7 is a bottom view of a conventional electronic component and FIG. 8 is a cross sectional view of FIG. 7.
In conventional electronic components, the coil 73 is wound around a toroidal core 72 and the leads of the coil 73 are connected to terminals 74, thereby, forming a coil element 71. This coil element 71 is wrapped in a resilient (elastic) protective material 78 such as silicon resin and is then housed in a resin case. Resin such as epoxy 79 is then injected into the case 75, thereby completely sealing the component. The toroidal core 72 and the external terminals 74 are fixed to the case 75 by this resin 79.
In such electronic components, the pressure at the time of injecting the resin 79 into case 75 and the pressure caused by contraction when the resin 79 solidifies is absorbed by the resilient protective material 78, thereby preventing degradation of the characteristics of the coil element 71. However, in the case of such conventional electronic component, it has been difficult to produce resilient (elastic) protective material 78 and resin 79 with equal thermal heat expansion coefficients. In general, the thermal expansion coefficient for the resilient protective material 78 is greater than that for the resin 79. Moreover, air bubbles sometimes enter the resilient protective material 78 when the coil element 71 is wrapped and remain after the resilient protective material 78 has solidified. Accordingly, in conventional electronic parts, bubbles within the resilient protective material 78 or the resilient protective material 78 itself become swollen by heat from solder when the external terminal 74 is soldered to the printed board of the electronic device so that the resin 79 or the case 75 swells to result in cracks in the resin 79 or the case 75. When the electronic component is thin, the thickness of the resin 79 sealing the resilient protective material 78 is also thin, and the space between the resin 79 and the connecting surface of the external terminal 74 is narrow. In such components, the swollen resilient protective material 78 can break the resin 79, and can be projected out from the connecting surface of the external terminal 74.